In the prior art, controlled release of soil nutrient additives has generally accomplished by use of fertilizer pellets in which a nutrient composition is either encapsulated within a special release control coating whose makeup differs from that of the interior of the pellet and/or in which the body of the pellet is powdered, granular, liquid, gel or semi-liquid bonding agent intended to maintain the structural integrity of the pellet.
For example, it is known to form agricultural pellets by blending various nutrient containing substances such as mineral slag, clays, mining waste processed sewerage sludge, urea, animal manure, phosphates and/or manufactured chemicals with various binders such as Portland cement, magnesium sulfate, lignin, Methacel® and/or organics such as sugar or molasses. While Portland cement is effective as a binding to reduce premature pellet disintegration, it is a relatively heavy material and contributes negligible, if any, nutrient value. The use of Portland cement, or other nutritionally “nonparticipating” binders, is undesirable from the standpoint that such materials entail costs not only for initial pellet production but also for packaging, handling, shipping and distribution in exchange for little, if any nutrient benefit. Accordingly, the economic efficiency of such agricultural pellets is somewhat undesirable.
As noted above, it is also known to coat fertilizer pellets with encapsulating materials such as sulfur or plastics which disintegrate at an at least somewhat predictable rate in the preserve of environmental factors such as water and/or biological agents typically present in said encapsulation and conventional incorporated binders. Both help, to maintain the shape of pellets while retarding the release of water-soluble nutrients contained within the pellet. See for example U.S. Pat. No. 3,630,713; U.S. Pat. No. 3,647,416; U.S. Pat. No. 4,023,955; U.S. Pat. No. 4,486,217; U.S. Pat. No. 4,082,533, and U.S. Pat. No. 5,030,267.
U.S. Pat. No. 6,939,387 describes the role of silicon (Si) in the growth of plants and the benefits of providing supplemental silicon sources, especially in the case of certain commercially important crop species, such as rice and sugar cane, which take in substantial amounts of silicon from the soil. Silicon supplementation through application of calcium silicate (CaSiO2), and magnesium-enhanced calcium silicate, has also been found to be highly beneficial in the cultivation of certain grasses such as St. Augustine grass, Bermuda grass, bent grass, rye grass, and other grasses used on golf courses, residential and commercial lawns. The '387 patent discloses a pelletized soil enhancer comprised of calcium silicate (CaSiO3) and magnesium sulfite (MgSO3) for increasing the amount of calcium, silica, magnesium, and potassium available for uptake by growing plants. The pellet may also include expandable clay which aids pellet disintegration upon exposure to sufficient moisture. The mechanical integrity of the pellets disclosed in the '387 patent is attributable principally to the inclusion of magnesium sulfite and/or organic binders.
Water-disintegrable fertilizer pellets formed from cement kiln dust and a water-soluble binder are disclosed in U.S. Pat. Nos. 5,743,934 and 5,997,599 to Womack et al. primary plant nutrients, secondary plant nutrients (magnesium, sulfur and/or calcium), and/or micronutrients (iron, copper etc.) may also be present in the pellets. At least 15% of the resulting pellet is from a calcium source, such as kiln dust, lime, limestone, or gypsum. This results in a pellet having only with a small amount of bio-available silicon. The small quantity of silica in the pellet provides insufficient silicon for efficient uptake of nutrients by various types of plants, particularly those requiring copious amounts of silicon. Consequently, application of large quantities of such pellets would be necessary to provide sufficient amounts of silica for such species.
Fertilizer pellets according to the prior art are commonly formed by blending nutrient-containing compounds with Portland cement, magnesium sulfate, lignin, and organics. The blend is fed to a rotating disc pelletizer together with sufficient moisture to form pellets of controlled size. The pellets are then cured and strengthened in a dryer. During drying, significant pellet attrition occurs as a result of mechanical breakage. As much as ten percent 10% of the pelletized feed material from the pelletizer may break during the drying operation alone. This substantial percentage of broken fragments is typically separated from the remaining unbroken pellets using a mesh screener. The fragments can be, and typically are, reprocessed but at the expense of both remanufacturing cost and decreased overall production yield.
Pellet breakage occurs during packaging, shipping, handling and end use application. Breakage occurring during any post-manufacturing stage interferes with efficient and uniform pellet application and compromises the designed time release characteristics of the pellet. Nutrient release by pellet fragments is accelerated to a degree which is relatively unpredictable depending on such variables as the number of broken pellets and applied to the crop and the size of those broken fragments. Fragmentation of pellets can also form dust which can become airborne and contaminate facilities and equipment. Such dust can also be inhaled if appropriate precautions are not taken.
Efforts to prevent pellet breakage by increasing the proportion of plasticized coatings, Portland cement or other conventional binders, and/or by adding thicker layers of encapsulating coating, in attempt to further strengthen the pellets, increases manufacturing cost and reduce the availability of nutrient content per unit weight of the pellets. Consequently, a greater overall weight of pellet material must be packaged, shipped handled and applied in order to deliver a given amount of nutrient to a given crop. Accordingly, if used at al, it is desirable to minimize the proportion of Portland cement or other nutritionally non-participating binders in an agricultural pellet.
Although prior art fertilizer pellets have generally been effective to some degree in retarding nutrient release, there is a need for an agricultural pellet which is not only effective to predictably control the release of nutrients but is also more economical to manufacture, package, ship and apply than agricultural pellets of the prior art.
There is also a need for an agricultural pellet which, in addition to the foregoing desirable characteristics also provides a significant source of bio-available silicon.
There is also a need for an agricultural pellet which not only provides all of the foregoing characteristics but also can provide for an accelerated release of nutrients through the incorporation of a fragmentation agent, such as an expandable clay.